1. Field of the Invention
This invention relates to a method of identifying the particles contained in a gaseous or liquid carrier medium. In particular, this invention relates to a method of identifying these particles by measuring scattered light through the medium to be analyzed.
2. The Prior Art
Scattered light measurement for identifying and classifying the particles contained in aerosols or transparent liquid is known. In these measurements, a light beam is aimed at the medium to be analyzed. The beam is scattered when it impacts a particle in the carrier medium, and is measured by a detector. Based on this data, it is possible to determine various properties of the particle, for example, its size, size distribution, shape and position.
Optical particle measurement is applied particularly in fields where examination of small particles is not possible using conventional methods. These known methods and apparatuses are largely based on Mie scattering theory. This is described in DE 43 41 573 C1, and has significant drawbacks. First, the optical measuring arrangement of this method detects only the forward scattering component. Second, the proposed astigmatic focusing causes a loss of intensity, so that small particles cannot be detected at all. Finally, since only one detector is employed, the geometric shape of the detected particles cannot be determined.
The method and device defined by DE 44 14 165 C1 measures light scatter on particles by arranging a plurality of pulsated light sources one after the other in a section of a tube, along with a common receiver. However, this solution also has several drawbacks. Since only one detector is employed, the scattered light produced by the interaction of the light with the particle can be detected only under a fixed space angle. Successive pulsating of the various light sources does result in different angles of incidence on the volume being measured. However, angular resolution of the measured light relative to the position of the particle cannot be measured, because different cross sections of scatter are measured with predominantly nonspherical particles due to the different angles of irradiation. Consequently, the size and shape of the detected particles cannot be correctly determined.
An inspection of a plurality of planes of scatter is possible by a particle measuring apparatus shown in EP 0 493 806 A2. With this apparatus, a ring-shaped arrangement of the detectors is employed for measuring larger particles, and an arrangement of detectors mounted at greater angles is used for measuring smaller particles. However, inspecting several planes with the ring-shaped detector arrangement is only achieved with forward scatter. As with the previously discussed prior art documents, only one plane is available for measuring larger angles of light scatter. This apparatus consequently does not permit any exact determination of the shape of particles. Moreover, this solution is afflicted with the shortcomings associated with only one light source.
U.S. Pat. No. 5,416,580 to Trainer, the disclosure of which is herein incorporated by reference, discloses a method and a device for identifying the particles contained in a gaseous or liquid carrier medium by measuring scattered light. In this patent, at least three light sources are aimed sequentially at the volume to be measured, and at least three detectors are used. The detectors are operated simultaneously and the measured scattered light values acquired by the detectors are correlated and evaluated. The goal of this invention is to expand the range of angular movement within one plane of scatter by sequentially switching the light sources on and off, with the light sources being arranged in one plane.
However, like the arrangement shown in EP 0 493 806 A2, this does not permit any angle-resolved measurement in different planes of scatter. Even with a suitable arrangement of light sources and detectors for detecting a plurality of planes of scatter, operating the light sources sequentially has decisive drawbacks. For example, the light sources have to be switched on and off very rapidly in order to fully measure the same particle in the measured volume. In the case of nonspherical particles, operating the light sources sequentially has the risk that, in addition to measurement of different particles, rotation of the particle in the measured volume may lead to an angle of incidence of the subsequent light source that is no longer definable, and thus subject to incorrect interpretation.
In addition, it is not possible to exactly determine the shape of the particles using the structure shown in U.S. Pat. No. 5,416,580. Furthermore, with spherical particles flowing at a high rate, no suitable measurement is possible for identifying particle size and refractive index, because it is not certain that the same particle is always present in the measured volume during the course of the measurement.
U.S. Pat. No. 3,822,095 to Hirschfeld describes an apparatus that uses a single light source. The light is split up by a specially designed optic into three different virtual light sources. The Hirschfeld system consists of three optical units (a detector and an apparent light source) looking at different measurement volumes. They are measuring the flow at the same sample but not at the same spot within the medium. To determine the shape and material dependent properties, it is necessary to illuminate the particle by three physically different light sources at the same time and the same spot. This is because evaluating shape and material properties requires simultaneous measuring in different scattering planes with respect to the orientation of the particle. This can only be done by measuring within one common measuring volume at the same time. Hirschfeld measures different properties of the particles at different locations and different times with three different apparent light sources generated from one optical light source.
U.S. Pat. No. 4,701,051 to Buchhave et al. discloses a laser doppler apparatus (LDA) that uses only one light source to generate two virtual coherent light sources to provide an interference pattern in the measurement volume. Thus, only one physical light source is used together with several detectors. The LDA apparatus is therefore measuring in only one scattering plane with one light source. The aim is to determine velocity and size of the spherical particles. The LDA method is not applicable to non-spherical particles. A measurement of shape and material properties is thus not possible with LDA.
U.S. Pat. No. 5,052,806 to Snyder et al. describes a system with only one light source as well. Here, the beam is split and a frequency shift is applied. This measurement method is state of the art in optical physics and is called heterodyne measurement. It is necessary to have the frequency shift and the light from one light source because this measurement method is based on the coherence of the two virtual light sources generated by one laser (so only one physical light source is used.) Therefore, this method is not measuring the light coming from three different light sources, hitting the particle at the same time in the same measuring volume to provide a measurement of the scattered light in different scattering planes.